It's complicated by the long and complex history of the stories that make up The Silmarillion. Some of them are among the oldest stories JRRT ever wrote, going back to the late 'teens (1915-1919). Jen can probably add more about this, but some of the stories went through 3 or more significant revisions; some of the stories existed as both brief overviews and very detailed narratives.
The reason for the many reworkings, revisions, and different narrative styles is that JRRT couldn't decide on a framework in which to present the stories; he went from tales told around a hearth fire (The Book of Lost Tales) to a more concise collection of legends and back again. All of that was complicated by the stories themselves changing over the 50+ years since he put the first versions on paper.
When I really got into reading The Book of Lost Tales earlier this year, I found the background history on the stories to be as interesting as the stories themselves. For instance, did you know in at least one version of the Tale of Beren and Luthien, Beren was an Elf? Amazing but true!
But then JRRT came to his senses. ;^)
Seriously, having Beren as an elf would take away from the story, wouldn't it?